A will can be amended or revoked by a testator. An amendement to a will is called a codicil. In order to revoke your will or create a codicil, a testator must meet the same requuirements as if he were creating a will. Specifically, a codicil must be in writing and signed by the testator. In order to revoke his will or create a codicil, a testator must understand the significance of his action and must not be unduly influenced or forced to do so by another person.
If you wish to make changes to your current will, do not write directly on your will. Writing in new clauses or scratching out parts of your will creates confusion about what you intended and may result in increased court involvement or fighting between your family members. It is clearer for your devisees and the court if you sign a codicil or an entriely new will in order to change your wishes.
(This is not intended to be legal advice and is only informational)